The discussion below is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.
Aspects of the present invention relate to a wind turbine comprising a generator with a generator rotor and a stator, with a gap between the stator and the generator rotor and with magnets and electro-conductive windings at opposite sides of the gap. Aspects of the invention also relate to a method of monitoring the length of the gap, particularly during operation of the wind turbine.
Generally, a wind turbine generator includes a rotor having two or three blades, although turbines with one blade or four or more blades also exist. The rotor is typically mounted to a shaft within a nacelle on top of a tower. The blades, attached to a rotatable hub, transform mechanical wind energy into a mechanical rotational torque that drives the generator rotor of a generator. The generator rotor can be coupled to the rotor through a gearbox or the coupling can be gearless, as in so-called direct-drive wind turbines. The generator converts the rotational mechanical energy to electrical energy, which is fed into a utility grid.
Generators typically comprise a generator rotor and a stator which are separated by a cylindrical air gap. During operation, a magnetic field, generated by a plurality of magnets, passes through a portion of the air gap. The magnets can, e.g., be permanent magnets, wound magnets mounted on the rotor, and/or currents induced in the rotor iron. The effective transmission of the magnetic field through the air gap is at least partly dependent on the length of the air gap. In this context, the length of the air gap is the radial distance between the rotor surface and the stator surface.
In operation, loads on the rotor are introduced by wind force via the blades. Such loads can deflect the generator rotor in such a way that the air gap length is changed or becomes nonuniform. If the radial air gap length becomes too small, the rotor and stator components can hit resulting in severe damage to the generator.
EP 1 870 566 A1 discloses a wind turbine with a generator comprising a stator and a rotor. The air gap between the stator and the rotor is measured by a number of sensors and adjusted by blade pitch control. During operation of the wind turbine, strong magnetic flux occurs in the air gap. This hinders accurate and reliable measurement of the air gap length. Moreover, since the sensors are located near the air gap, the sensors cannot be easily replaced.